Metalloporphyrins are photosensitizers of methylviologen (MV.sup.2+) reduction. The reduced methylviologen reduces water to hydrogen gas in the presence of a colloidal platinum catalyst. A system which is composed of a metalloporphyrin, the oxidized methylviologen, the platinum catalyst, and a sacrificial electron donor provides a method of collecting solar energy and storing that energy as a gaseous fuel.
Previously, metalloporphyrins with positively charged substituents at the periphery of the porphyrin ring have been found to be effective in photosensitizing the methylviologen reduction. Quantum yields as high as 0.75 are possible with the most positively charged porphyrins. However, the efficiency of the photoreduction of methylviologen decreases as the number of negatively charged groups on the ring increases.
Japanese Published Application (JP No. 100496) discloses the use of a metal porphyrin in the light-induced production of hydrogen. The metal porphyrin, which is in solution with an electron donor (e.g., EDTA), and a platinum metal catalyst, is subjected to light waves which ultimately produce hydrogen.
U.S. Pat. No. 4,325,793 to Kisch discloses a method for splitting water with light to produce hydrogen by use of a special catalyst.
U.S. Pat. No. 4,382,846 to Gratzel et al and U.S. Pat. No. 4,211,621 to Porter teach the general reaction employed in the light-induced production of hydrogen using a photosensitizer.
Fuhrhop et al, Liebigs Ann. Chem., 1983, 204-210 disclose that reduced methylviologen can be formed by photochemical reduction in the presence of EDTA with water-soluble metalloporphyrins of different oxidation potentials, namely zinc and tin(IV) complexes. Hydrogen is produced in the presence of a collodial platinum catalyst.